1. Field of the Invention
The present invention relates generally to marine transportation of liquids. In another aspect, the invention concerns ocean-going vessels for transporting liquefied natural gas (LNG) over large distances.
2. Description of the Prior Art
Vessels designed to carry liquefied natural gas (LNG) are among the most expensive commercial cargo-carrying vessels in the world. This is primarily due to the relatively light weight of LNG (requiring a large volume for a given weight of cargo) and the extremely low temperature required to keep the LNG in its liquid state under the low pressures necessary to enable long at-sea transit of commercially viable LNG quantities. LNG is typically transported at or slightly above atmospheric pressure and at a temperature of approximately −260° F. (−160° C.). All LNG containment systems (i.e., tanks) must be constructed of materials which can withstand the extremely low temperatures and the wide temperature changes from ambient conditions to in-service conditions. Further, all tanks must provide effective temperature insulation to prevent heat inflow and unacceptable cooling of the vessel's basic hull structure.
Conventional tanks for carrying LNG aboard ocean-going vessels generally fit into one of the following two categories: (1) “independent tanks,” which are generally self-supporting and rely only upon foundations to transmit the gravitational and other forces of their weight and the weight of their contents to the surrounding hull structure; and (2) “membrane tanks,” which rely entirely upon the surrounding hull structure to maintain their shape and integrity and to absorb all of the hydrostatic forces imposed by their contents. Membrane tanks are generally constructed of either stainless steel or Invar (a high nickel content alloy with minimal thermal expansion characteristics). Membrane tank systems include load-bearing thermal insulation that can transmit the hydrostatic and hydrodynamic loads to the hull structure.
A large percentage of LNG tanker-ships in use today include several independent, free-standing spherical tanks lined up along the length of the ship. Each spherical tank is supported by a cylinder or circular ring that is in turn supported by the bottom of the ship's hull. Spherical tanks, while attractive from the standpoint of maximizing volume-to-surface ratio and equalizing stresses over the surface, have serious drawbacks as cargo tanks. For example, the shape of a spherical tank does not match the shape of the tanker-ship, thereby resulting in wasted space in the hull. This void space near the bottom of the hull forces the center of gravity of the ship upwardly, thereby destabilizing the ship. Spherical tanks typically extend above the deck of the ship, which can dramatically reduce the amount of horizontal deck space available to supporting mooring equipment and other equipment. In addition, the spheres themselves are not free-standing, and so free-standing spherical tank systems include a significant support system. This support system adds both to the cost and the weight of the overall containment system.
Prismatic tanks avoid some drawbacks of spherical tanks. A “prismatic” tank is a tank that is shaped to follow the contours of the ship's hull. At midship the tanks may be in the shape of rectangular solids, with six flat sides (four vertical sides, a top side, and a bottom side). They may also have flat sides that converge downwardly to better match the hull. Free-standing prismatic tanks make more efficient use of below-deck volume than do spherical tanks. However, prismatic tanks contribute significantly to weight and cost because they employ heavy plates and a considerable amount of bracing to keep the plates from distorting under load. Some conventional LNG tanker-ships employ prismatic membrane tanks. Prismatic membrane tanks offer the same space efficiency advantages as independent prismatic tanks, but are typically much lighter than free-standing tanks.
When LNG is carried in a tanker-ship, sloshing of the LNG can be problematic because it increases the hydrodynamic loads on the tank, decreases the stability of the ship, and promotes vaporization of the LNG. Sloshing is cause by the movement of the ship and the existence of free surface area of the LNG. Sloshing could be substantially eliminated if it were possible to completely fill the tank with LNG. However, conventional practice is to fill LNG tanks to a maximum of about 98.5% of their full capacity so as to allow for expansion. In addition, it is not economically feasible to fill LNG tanks to 100% capacity because doing so would require a significant decrease in the fill rate of the tank during filling of the final 1–2% of capacity. This decrease in flow rate is required in order to avoid rapid over pressurization of the tank and/or overfilling and leakage through the venting or other systems. The filling of conventional LNG tanks to less than 100% capacity leaves a void space between the surface of the LNG and the top of the tank The resulting free surface area of the LNG allows sloshing to occur and promotes vaporization of the LNG. One way to inhibit sloshing in LNG tanks is to equip the tank with internal baffles. However, the use of anti-sloshing baffles increases the material, construction, and maintenance costs of the tank.